Residential circuit arc detection

ABSTRACT

An electronically controlled circuit breaker for detecting arc and a method for detecting arc conditions are disclosed. The circuit breaker and method uses digital Fourier transformation on sampled line current signals and bins the even, odd, and fundamental transforms. Even arc signals and non-harmonic arc signals are calculated from the values in the bins and used within an expert arc algorithm which uses an accumulator to sum an increment based on whether the even arc signals and non harmonic arc signals exceed a fixed threshold. A trip signal is issued when the increment exceeds a predetermined value.

BACKGROUND OF THE INVENTION

This invention relates generally to residential circuit arc detection,and, more particularly, this invention relates to arc detectionemploying frequency analysis which does not give false indications underthe wide variety of “normal” loads.

U.S. Pat. Nos. 5,659,453 and 5,578,931 to Russell describe systems forlow level arc detection in electrical distribution circuits. Thesesystems do not eliminate the odd harmonics, in fact, they incorporatethem into the arc detection. This works in utility distribution circuitswhich supply many homes, commercial and industrial establishments. Insuch circuits, the current sums many linear and non-linear loads. Aslinear (primarily fundamental) loads dominate the normal current, theappearance of odd harmonics can signal arcing in a distribution circuit.In a residence, however, some circuits have non-linear loads like lightdimmers which provide significant odd harmonics and are always present.As such, odd harmonics do not distinguish arcs from loads. These systemsalso disclose an approach to collection of non-harmonic energy by way ofinteger+half (30, 90, 150, 210 Hz, etc.) to represent non-harmonicenergy. Unfortunately, this approach ignores most of the non-harmonicfrequency band.

The prior art also employs time domain procedures which attempt to usefilters and heuristic methods (from examining arcs) to distinguisharcing from non-linear loads.

BRIEF SUMMARY OF THE INVENTION

The above discussed and other drawbacks and deficiencies are overcome oralleviated by an electronically controlled circuit breaker comprising aline current sensor sensing line current signals, a conditioning circuitfor splitting the line current signals into a first signals and a secondsignals, a high pass filter for passing amplified first signals, ananalog to digital converter for converting the current signals intobinary form, a processor for receiving the binary form of the currentsignals, wherein the processor determines the fundamental frequency ofthe current signals, processes the first multiples of the fundamentalfrequency, and squares and sums the multiples to yield even, odd, andfundamental values, even, odd, and fundamental bins within the processorfor receiving the even, odd, and fundamental values, wherein theprocessor processes even arc signals and non-harmonic arc signals fromthe even, odd, and fundamental values in the bins, and an expert arcalgorithm within the processor having an accumulator for calculating anincremental value based on even arc signal and non-harmonic arc signalinput and a fixed threshold for which to compare the incremental value,wherein the processor issues a trip signal when the fixed threshold isexceeded by the incremental value.

Similarly, a method for detecting arc conditions in anelectronically-controlled circuit breaker comprises detecting linecurrent signals with a line current sensor, splitting the line currentsignals with a conditioning circuit into first line current signals andsecond line current signals, amplifying the first line current signals,passing the amplified first line current signals through a high passfilter, sending the first line current signals from the high pass filterand the second line current signals to an analog to digital converter,converting the line current signals into binary form in the analog todigital converter, determining the fundamental frequency of the firstline current signals in a processor and processing the first sixteenharmonics of the fundamental frequency, providing even, odd, andfundamental bins within the processor for binning the fundamentalfrequency, even harmonic, and odd harmonic values, creating a meansquare value from the second line current signals, processing even arcsignals and non-harmonic arc signals from the values in the bins,calculating an incremental value based on the even arc signals andnon-harmonic arc signals, comparing the incremental value to a fixedthreshold, and issuing a trip signal indicating an arc fault conditionwhen the fixed threshold is exceeded by the incremental value.

The above-discussed and other features and advantages of the presentinvention will be appreciated and understood by those skilled in the artfrom the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the FIGURES wherein like elements are numbered alike in theseveral FIGURES:

FIG. 1 shows a circuit diagram of an electronically-controlled circuitbreaker;

FIG. 2 shows a block diagram of the major elements occurring in theoverload, ground fault, and arc fault detector block of FIG. 1;

FIG. 3 shows a block diagram of the digital signal processing thatoccurs in FIG. 2; and,

FIG. 4 shows a block diagram of an arc algorithm that is used followingthe digital signal processing shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an example of an electronically-controlled circuit breakerwith time overcurrent arcing and instantaneous protection and groundfault protection. In this application, the circuit breaker 10 connectsto a 120 Volt source 12. After the circuit breaker contacts 14, the linecircuit 16 is wound around the AC solenoid 18 for several turns. Thisprovides a current-based instantaneous trip for bolted faults at thecircuit breaker 10 which do not allow the electronic circuits to poweron. At this point, the line 16 is tapped and connected to the solenoidcoil 20.

The solenoid 18 connects to a bridge rectifier 22. A bridge rectifieruses diodes to generally convert ac voltages into dc voltages. The basicprinciple of a diode is that current can flow through in one direction,from cathode to anode, but not in the other. Practically speaking,diodes have some resistance when forward biased, but the value is verylow. Similarly, some current flows through a diode when it isreverse-biased, but the resistance is so high the current will be of anegligible value. A bridge rectifier uses the entire input cycle, and iseasy to filter. A bridge rectifier requires four diodes as shown. At anypoint of the input cycle, two of the diodes in a bridge are conductingand two are reverse biased.

The positive output of the bridge rectifier 22 connects to the lowvoltage power supply 24 and a silicon controlled rectifier “SCR” 26which is used to operate the solenoid 18. An SCR is a special purposesemiconductor device which uses a similar symbol as a diode with theaddition of a third lead, called a gate. If a voltage is applied betweenthe cathode and the anode, but the gate is at zero volts (grounded), nocurrent will flow through an SCR. If a voltage greater than somespecific value is applied to the gate, current will start to flow fromcathode to anode against only a small internal resistance (as with anordinary diode). This current will continue to flow—even if the voltageon the gate is removed. The only way to stop the current flow through anSCR once it has started is to decrease the positive voltage on the anode(or remove it altogether). When the anode voltage drops below apredetermined level, current flow will be blocked, even if the anodevoltage returns to its original value, unless the gate receives therequired triggering voltage.

The other AC input to the bridge rectifier 22 is connected to neutral 28to complete the circuit. The load 30 is where the electrical energy isusefully employed, that is, where the energy is converted fromelectrical form into some other form.

An important feature of the present invention occurs in the center block32 where the protection functions (Overload, Ground Fault and Arc FaultDetector) are represented. This block receives power from the powersupply 24, and signals from 2 current sensors 34, 36 where sensor 34senses line current and sensor 36 senses the differential currentbetween line 16 and neutral 28. The block 32 includes a dormantoscillator 38 (where an oscillator is an electronic circuit thatproduces a repeating ac signal), which uses the 7 KHz neutral coil 40 toinject a ground current on the neutral line 28 when the neutral 28 andground lines are shorted. Ground wires (not shown) are attached to theneutral wires in the load center 30 between the circuit breaker 10 andthe utility source 12. By code, the ground wire, which runs in the samesheath as the neutral and protected line wires, should never beconnected to the neutral wire downstream of the circuit breaker 10(between the circuit breaker 10 and the load 30). The dormant oscillatorcircuit 38 senses when this condition (which can defeat ground faultprotection) occurs. The block 32 also provides a trip signal 42 to theSCR 26 to energize the solenoid coil 20 and operate the circuit breaker10 when an arcing, time overcurrent, instantaneous or ground faultcondition occurs.

FIG. 2 shows the major elements of the center block 32 in FIG. 1. Simpleconditioning circuits split the line current signal 50 from the linecurrent sensor 34 into two signals 54, 56 where signals 54 are amplifiedand passed through a high pass filter 44. A high pass filter allows highfrequencies (frequencies greater than the fundamental) to pass throughto the output, but increasingly blocks lower frequencies. These signals54, 56 from the line current sensor 34 and the ground fault signal 52from the ground fault current sensor 36 are sampled in an analog todigital converter 46 at multiple, preferably at least 32 times, thefundamental frequency. An analog to digital converter accepts a voltageand gives an output number in binary form proportional to the voltage.The primary frequency is called the fundamental frequency, or justfundamental, and the multiples of the fundamental are called harmonics.The line current signals 54, 56 are passed to an arc detector block 48which provides detection and tripping of the circuit breaker 10 due toarc faults.

As part of the arc detection, the mean square or “MS” of the linecurrent is generated and passed to the time overcurrent section 58 forconventional electronic overcurrent protection. Methods for usingsampled data as overcurrent protection exist in the art including U.S.Pat. No. 4,589,052, hereby incorporated by reference. Similarly, thesampled ground fault signals 52 provide input to a conventionalelectronic ground fault protection represented at block 60. The raw linecurrent samples 56 are processed by a peak detecting algorithm toprovide instantaneous tripping represented at block 62. Trip conditionsfrom any of these four elements are combined in a logical ‘OR’ circuit64 (gate) to operate the trip solenoid 18 by pulsing the SCR device 26.The SCR 26 conducts current from the line 16 through the trip solenoidcoil 20 and bridge rectifier 22 and finally to the neutral circuit 28.These elements provide the full breaker function and are required aspart of a usable circuit breaker 10.

The line current values 54, 56, after passing through the analog todigital converter 46, are employed in an “Instantaneous OverCurrent”calculation (“IOC” as shown in FIG. 3). Such a calculation is part of abasic circuit breaker function.

A main feature of the present invention is the arc detection 48 andexpert arc tripping algorithm 66. FIG. 3 shows a block diagram of arcdetection 48, in particular, the digital signal processing (from thedigital signal processor 68 or DSP) that occurs in FIG. 2. As discussedwith respect to FIG. 2, the input signals 50 of line current from theline current sensor 34 are sampled. Sampling is where signals are notmeasured or specified continuously, but rather at fixed intervals oftime. Thus, where the signals are measured only at the discrete times,then the discrete Fourier Transform represented by block 70 isprocessed. The high pass samples 54 (frequencies>fundamental) areprocessed in a microcomputer 68 to yield the discrete Fourier componentsof the first 16 harmonics of the fundamental (that is, the first 16multiples of the fundamental frequency). Although the first 16 harmonicsare preferred, it would be within the scope of this invention to employa differing number of harmonics. These amplitude values are squared andsummed into three ‘bins’ for each cycle, the even bin 72 (2^(nd),4^(th), 6, 8, 10, 12, 14, and 16^(th)), the odd bin 74 (3^(rd), 5^(th),7, 9, 11, 13, 15), and the fundamental bin 76(1^(st)).

Separately, the unfiltered line current samples 56 are squared andsummed to create a mean square (MS) value 78 of the total current. Thefundamental signal 76 is also drawn from this unfiltered signal 56 viadiscrete Fourier transformation 70. The non-harmonic signal 80 isachieved by subtracting the sum of squares of the odd 74, even 72, andfundamental 76 harmonics from the sum of squares 78 of the line current56.

Note that no anti-aliasing filter has been applied. By definition, suchfilters modify the frequency structure of the sensed signal, which isnot a desirable or necessary feature for this invention. A unique ideain this concept, which involves binning of even, odd, and non-harmonicenergy, is that the higher order frequencies will alias into the properbins. Arc energy can be of a higher frequency, but conducted currents,both fundamental and harmonic, tend to drop off dramatically above the13^(th) harmonic. Eliminating the anti-aliasing filter and selecting anexact multiple of the fundamental frequency allows energy from thesehigher order frequencies to be collected in the proper frequency bins:odd, even, and non.

The interim result of these calculations is two values, both singlecycle mean square, of the even harmonics 82 and the non-harmonics 80(energy in the waveform not occurring at harmonics of the fundamental).A major premise of this invention is that the even 82 and non-harmonics80 are uniquely indicative of arcing conditions.

These signals may occur in small quantities naturally (dither in adimmer circuit will generate even harmonics). Both signals are passedthrough multiple cycle integrators 84, 86 to average the backgroundnoise. This noise level is subtracted from the present signal. Thisresults in the EA 88 (even arc) and NHA 90 (non-harmonic arc) signals.Both of these signals 88, 90 enter the expert arc tripping algorithm 66as well as the current mean square current (MS) 92 used as a qualifierthere.

The results of the integrated means square (MS) value 78 are fed to aTime Over Current calculation (“TOC”) which is also part of a basiccircuit breaker function.

FIG. 4 shows generally what occurs in the expert arc tripping block 66.The amplitude of the EA 88 and NHA 90 signals are compared against athreshold which is a function of the mean square line current. Thisprovides a final security level to avoid nuisance tripping. The expertarc accumulator 100 adds a value which follows:

Increment=EA*T 1+NHA*T 2+n*(EA+NHA)*(T 1*T 2)−m*(notT 1*not T 2)

where T1 and T2 are binary signals (values 1 or 0) which result from thecomparison of the EA signal (T1) and NHA signal (T2) with a submultipleof the MS or mean square signal. As the basic load current (MS)increases, some amount of even (EA) and non (NHA) harmonic signal occursnaturally. The comparator elements that produce T1 and T2 will notproduce a “1” unless the corresponding signal is greater than someproportion of the load current. Thus, in the equation, the signals EAand NHA are multiplied either by “1” or “0”. This has the effect of notletting the EA or NHA signal pass to the final accumulator 100.

The above-described equation adds counts from either the even ornon-harmonic input if the corresponding threshold has been exceeded.Threshold T1 is a binary signal which represents that the even harmonicsignal EA is greater than a proportion of the load current MS. ThresholdT2 is a binary signal which represents that the non-harmonic signal NHAis greater than a proportion of the load current MS. If both thresholdsare exceeded, an acceleration factor n is applied to increase the tripcount faster. If neither threshold is exceeded, the count is decreasedby a linear factor m.

When the accumulator 100 exceeds a fixed threshold, the trip signal 42is given.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. An electronically controlled circuit breakercomprising: a line current sensor sensing line current signals; aprocessor for determining the fundamental frequency of the currentsignals, wherein the processor processes a preselected number ofmultiples of the fundamental frequency, and squares and sums themultiples to yield even, odd, and fundamental values; even, odd, andfundamental bins within the processor for receiving the even, odd, andfundamental values, wherein the processor processes even arc signals andnon-harmonic arc signals from the even, odd, and fundamental values inthe bins; and, an expert arc algorithm within the processor having anaccumulator for calculating an incremental value based on even arcsignal and non-harmonic arc signal input, and additionally wherein theprocessor applies an acceleration factor to the incremental value whenboth an even arc threshold and a non-harmonic arc threshold are exceededby the even arc signal and the non-harmonic arc signal, respectively,and a fixed threshold for which to compare the incremental value;wherein the processor issues a trip signal when the fixed threshold isexceeded by the incremental value.
 2. The electronically controlledcircuit breaker of claim 1 further comprising a pair of separablecontacts and a solenoid.
 3. The electronically controlled circuitbreaker of claim 2 further comprising a bridge rectifier connected tothe solenoid.
 4. The electronically controlled circuit breaker of claim3 further comprising a silicon controlled rectifier connected to thebridge rectifier, wherein the, silicon controlled rectifier receives thetrip signal from the processor and energizes the solenoid to separatethe separable contacts when a tripping condition occurs.
 5. Theelectronically controlled circuit breaker of claim 1 further comprisinga neutral sensor.
 6. The electronically controlled circuit breaker ofclaim 5 further comprising a dormant oscillator for receiving signalsfrom the neutral sensor.
 7. The electronically controlled circuitbreaker of claim 1 further comprising a ground fault sensor for sensingdifferential current signals between line and neutral.
 8. Theelectronically controlled circuit breaker of claim 7 further comprisingmeans for detecting ground fault conditions, means for detectingovercurrent conditions, and means for detecting line current peaks forinstantaneous protection.
 9. The electronically controlled circuitbreaker of claim 8 further comprising a logical OR circuit for receivingtrip condition notification from any of the means for detecting groundfault conditions, means for detecting overcurrent conditions, means fordetecting line current peaks, and the expert arc algorithm, wherein,upon receipt of a trip condition notification, the logical OR circuitsends the trip signal.
 10. The electronically controlled circuit breakerof claim 1 further comprising a conditioning circuit for splitting theline current signals into first signals and second signals.
 11. Theelectronically controlled circuit breaker of claim 10 further comprisinga high pass filter for passing amplified first signals.
 12. Theelectronically controlled circuit breaker of claim 11 further comprisingan analog to digital converter for converting the current signals intobinary form.
 13. The electronically controlled circuit breaker of claim1 wherein the processor decreases the incremental value if neither theeven arc threshold nor the non-harmonic arc threshold is exceeded. 14.The electronically controlled circuit breaker of claim 13 wherein theprocessor decreases the incremental value by a linear factor whenneither the even arc threshold nor the non-harmonic threshold isexceeded.
 15. The electronically controlled circuit breaker of claim 1wherein the processor multiplies the acceleration factor to a sum of theeven arc signal and the non-harmonic arc signal when both the even arcthreshold and the non-harmonic arc threshold are exceeded.
 16. Anelectronically controlled circuit breaker comprising: a line currentsensor sensing line current signals; a processor for determining thefundamental frequency of the current signals, wherein the processorprocesses a preselected number of multiples of the fundamentalfrequency, and squares and sums the multiples to yield even, odd, andfundamental values; even, odd, and fundamental bins within the processorfor receiving the even, odd, and fundamental values, wherein theprocessor processes even arc signals and non-harmonic arc signals fromthe even, odd, and fundamental values in the bins; and, an expert arcalgorithm within the processor having an accumulator for calculating anincremental value based on even arc signal and non-harmonic arc signalinput and a fixed threshold for which to compare the incremental value;wherein the incremental value is calculated using the equation:Increment=(EA*T 1)+(NHA*T 2)+(n*(EA+NHA)*(T 1*T 2))−(m*(not T 1* not T2)) where thresholds T1 and T2 are binary signals which represent thatthe EA even harmonic signal or NHA non harmonic signal, respectively, isgreater than a predetermined proportion of load current of the circuitbreaker and where n is an acceleration factor applied when boththresholds T1 and T2 are exceeded and m is a linear factor used todecrease the Increment when neither threshold T1 nor T2 is exceeded; andwherein the processor issues a trip signal when the fixed threshold isexceeded by the incremental value.
 17. A method for detecting arcconditions in an electronically-controlled circuit breaker, the methodcomprising: providing an even arc threshold and a non-harmonic arcthreshold; calculating an incremental value based on even arc signalsand non-harmonic arc signals, wherein calculating the incremental valuecomprises comparing the even arc signals with the even arc threshold andthe non-harmonic arc signals with the non-harmonic arc threshold,applying an acceleration factor to the incremental value if both theeven arc threshold and the non-harmonic arc threshold are exceeded, anddecreasing the incremental value if neither the even arc threshold northe non-harmonic arc threshold is exceeded; comparing the incrementalvalue to a fixed threshold; and, issuing a trip signal indicating an arcfault condition when the fixed threshold is exceeded by the incrementalvalue.
 18. The method of claim 17 further comprising, prior tocalculating an incremental value: detecting line current signals with aline current sensor; determining the fundamental frequency of the linecurrent signals in a processor and processing a preselected number ofharmonics of the fundamental frequency; providing even, odd, andfundamental bins within the processor for binning the fundamentalfrequency, even harmonic, and odd harmonic values; and, processing evenarc signals and non-harmonic arc signals from the values in the bins.19. The method of claim 18 further comprising, subsequently detectingline current signals, splitting the line current signals with aconditioning circuit into first line current signals and second linecurrent signals, amplifying the first line current signals, and passingthe amplified first line current signals through a high pass filter. 20.The method of claim 19 further comprising creating a mean square valuefrom the second line current signals.
 21. The method of claim 20 whereincreating a mean square value from the second line current signalscomprises squaring each second line current signal and summing eachsquared second line current signal with additional squared second linecurrent signals for each cycle of line current signals.
 22. The methodof claim 20 further comprising comparing amplitudes of the even arcsignals and the non-harmonic arc signals against a threshold which is afunction of the mean square value.
 23. The method of claim 17 furthercomprising detecting ground fault conditions, overcurrent conditions,and line current peaks, notifying a logical OR circuit of a tripcondition, and issuing a trip signal upon notification of a tripcondition from either ground fault conditions, overcurrent conditions,line current peaks, or arc fault conditions.
 24. The method of claim 23further comprising pulsing a silicon controlled rectifier upon receiptof the trip signal.
 25. The method of claim 18 further comprising,subsequently providing even, odd, and fundamental bins, squaring eachvalue in the bins and adding the squared value in the bins withadditional squared values for each cycle of line current signals. 26.The method of claim 18 wherein processing even arc signals andnon-harmonic arc signals from the values in the bins comprises passingeven harmonic signals and non harmonic signals through multiple cycleintegrators to average background noise.
 27. The method of claim 17wherein calculating the incremental value comprises employing theequation: Increment=(EA*T 1)+(NHA*T 2)+(n*(EA+NHA)*(T 1*T 2)−(m*(not T1* not T 2)) where thresholds T1 and T2 are binary signals whichrepresent that the EA even harmonic signal or NHA non harmonic signal,respectively, is greater than a predetermined proportion of load currentof the circuit breaker and where n is an acceleration factor appliedwhen both thresholds T1 and T2 are exceeded and m is a linear factorused to decrease the Increment when neither threshold T1 nor T2 isexceeded.